Default physcial values and rounding of coords

There were really no reason for all the "if" checking before changing
the x and y values of the 2d component. It only slowed things down. The
rounding is necessary, because crafty doesn't do it iself, so it expects
to get integer numbers (otherwise you get tracers and other graphical
glitches). Same goes for the angle.

I also added default values for density, friction and restituion. Just
like in the current release version.

The examples "box2d" and "collision" were rewritten to work with the new
api.

The newer version of coffee shop (1.3.1 before, 1.6.3 now) resulted in
some small differences all over the craftybox.js.